Semiconductor memory device having voltage booster circuit

Abstract: An equalizing circuit (14) is connected between a pair of bit lines (BL, BL ). The equalizing circuit (14) is made up of three MOS transistors (11, 12, 13), and an equalization control signal (EQL) is supplied to the gates of the MOS transistors (11, 12, 13). A sense amplifier circuit (15) is connected to the bit lines (BL, BL ). The sense amplifier circuit (15) amplifies the potential difference occurring between the bit lines (BL, BL ), for the detection of data. The equalization control signal (EQL) is output from a level conversion circuit (16). An internal boosted voltage-generating circuit (17) constantly generates a boosted voltage (Vint) which is higher than an externally-applied power supply voltage (Vcc) applied to a power supply terminal (10). The boosted voltage (Vint) is applied to the level conversion circuit (16). The level conversion circuit (16) converts an input control signal (EQL'), whose high-level voltage is equal to, or lower than the externally-applied power supply voltage (Vcc), into the boosted voltage (Vint), thereby generating the equalization control signal (EQL).

Description: The present invention relates to a semiconductor memory device comprising a bit line-equalizing circuit which charges a pair of bit lines to predetermined potential levels and equalizes the potentials of the bit lines to each other before data is read out from a memory cell. In particular, it relates to a semiconductor memory device comprising: first and second bit lines; a boosted voltage generating circuit (17) for generating a boosted voltage (Vint) having a voltage level higher than a voltage level of a power supply voltage (Vcc) supplied to an input thereof; and a charging/equalizing circuit responsive to a charging/equalizing control signal for charging said first and second bit lines with a charging voltage and for equalizing said first and second bit lines;at the same equalized voltage level. Such a semiconductor memory device is disclosed in GB-A-2 118 795. However, this reference does not disclose how the boosted voltage is generated. EP-A-0 092 809 discloses a circuit for selectively switching a boosted voltage which is generated constantly in a boosted voltage generating circuit in order to solve the problem of delay of the boosted voltage. This reference, however, does not disclose or suggest any technical idea of using a boosted voltage which is constantly generated to be fed to a charging/equalizing control signal. EP-A-0 535 325 discloses a circuit for supplying a constantly boosted voltage to a word line. This reference, however, does not disclose the use of a constantly boosted voltage to be fed to a charging/equalizing control signal. In a dynamic random access memory (DRAM), an equalizing operation is performed before data is read out from a memory cell. The equalizing operation is an operation for charging a pair of bit lines to predetermined potential levels and then equalizing the potential levels to each other. In an ordinary case, the equalizing operation is performed by means of a bit line-equalizing circuit made up of N-channel MOS transistors. FIG. 1 is a circuit diagram showing the peripheral circuit configuration of a bit line-equalizing circuit employed in a conventional DRAM. (The bit line-equalizing circuit will be hereinafter referred to simply as an "equalizing circuit".) Referring to FIG. 1, an equalizing circuit 14 is made up of three N-channel MOS transistors 11, 12 and 13 and is connected between a pair of bit lines BL and BL . A sense amplifier circuit 15 for detecting data is also connected to the bit lines. In the equalizing circuit 14, two MOS transistors 11 and 12 are turned on in response to an equalization control signal EQL, and bit lines BL and BL are pre-charged in accordance with a pre-charging potential VBL. In response to the equalization control signal EQL, MOS transistor 13 is also turned on. Since the bit lines BL and BL are short-circuited to each other by the ON-state MOS transistor 13, the potentials of the bit lines BL and BL are equalized to each other. After the equalizing circuit 14 sets the potential of the bit lines in this manner, data is read out from a memory cell (not shown). The potential difference which is produced between the bit lines BL and BL when the data is read out is amplified by the sense amplifier circuit 15, so as to detect the data. In the conventional art, a signal whose level is equal to that of an externally-applied power supply voltage is used as the equalization control signal EQL supplied to the equalizing circuit 14. Alternatively, a signal whose level is temporarily raised by a known boot-strap circuit up to a level higher than that of the externally-applied power supply voltage is used as the equalization control signal EQL. The reason why the voltage level of the equalization control signal EQL must be raised by means of the boot-strap circuit is that, if that voltage level is low, a potential difference is produced between the source and drain of each of the N-channel MOS transistors 11, 12 and 13, and this potential difference does not permit the bit line to be pre-charged to a predetermined potential level VBL. FIG. 2 is a circuit diagram showing the peripheral circuit configuration of an equalizing circuit of another conventional DRAM. This DRAM is a so-called shared sense amplifier type wherein a sense amplifier circuit 15 is shared by two bit line pairs (BL1, BL1 ) and (BL2, BL2 ). In this type of DRAM, switch circuits 18a and 18b are formed between the bit line pairs and the sense amplifier circuit 15, so as to disconnect the bit line pairs from the sense amplifier circuit 15. Each of the switch circuits 18a and 18b is made up of two N-channel MOS transistors 19 and 20, and the gates of these MOS transistors 19 and 20 are controlled by switch control signals T1 and T2. The switch circuits 18a and 18b selectively operate when the potential difference between the paired bit lines is amplified by the sense amplifier circuit 15 for data detection. When the potential difference between bit line pair BL1 and BL1 is amplified, the two MOS transistors 19 and 20 of switch circuit 18a are turned on by switch control signal T1. Likewise, when the potential difference between bit line pair BL2 and BL2 is amplified, the two MOS transistors 19 and 20 of switch circuit 18b are turned on by switch control signal T2. In the DRAM depicted in FIG. 2, either signals whose levels are the same as that of an externally-applied power supply voltage or signals whose levels are temporarily raised to be higher than the level of the externally-applied power supply voltage by means of a known boot-strap circuit, are used not only as signals EQL1 and EQL2 but also as signals T1 and T2. The reason is that signals T1 and T2 must be high-level voltages when the MOS transistors 19 and 20 of switch circuits 18a and 18b are turned on. If signals T1 and T2 are not high-level voltages then, a potential difference is produced between the source and drain of each of MOS transistors 19 and 20, and the sensing margin is adversely affected thereby. To solve this problem, not only the voltages of signals EQL1 and EQL2 but also those of signals T1 and T2 are temporarily boosted by means of the boot-strap circuit. In some cases, use is made of a semiconductor memory device (such as a DRAM) which employs an internal voltage-generating circuit for generating an internal voltage different from an externally-applied power supply voltage. As such an internal voltage-generating circuit, there is proposed a type which employs a charge pump circuit to generate a boosted voltage at all times. However, in the case where a voltage boosted by the charge pump circuit is used as an internal voltage, a very large capacitance must be connected to the output terminal of the internal voltage generating circuit, so as to absorb the voltage variations during operation. The present invention has been developed in consideration of the above circumstances, and an object of the present invention is to provide a semiconductor memory device which enables easy control of the timing at which either an operation for equalizing potentials of bit lines or an operation for controlling the connection between the bit lines and a sense amplifier is performed, which ensures a high-speed operation, and which enables bit lines to be pre-charged to a predetermined potential level, as in the conventional art, without adversely affecting the sensing margin. The present invention provides a semiconductor memory device as initially defined, wherein the boosted voltage generating circuit is arranged to generate said boosted voltage constantly and the charging/equalizing control signal has a first voltage level of the boosted voltage supplied from said boosted voltage generating circuit, and the boosted voltage generating circuit includes: a reference voltage generating circuit for generating a reference voltage; a boosting circuit responsive to a boosting circuit control signal for generating the boosted voltage by boosting the power supply voltage; a voltage converting circuit for converting the boosted voltage to a converted voltage having a voltage level lower than the voltage level of the boosted voltage; and a voltage comparing circuit for comparing the reference voltage and the converted voltage to generate a comparison signal, and for outputting the comparison signal as the boosting circuit control signal. This invention will be better understood by referring, by way of example, to the accompanying drawings, in which: FIG. 1 is a circuit diagram showing part of a conventional DRAM; FIG. 2 is a circuit diagram showing part of a conventional shared sense amplifier type DRAM; FIG. 3 is a circuit diagram showing part of a DRAM according to the first embodiment of the present invention; FIG. 4 is a block diagram showing the configuration of an internal boosted voltage-generating means used in the DRAM depicted in FIG. 3; FIG. 5 is a circuit diagram showing detailed configurations of a reference voltage-generating circuit, a voltage conversion circuit and a voltage comparison circuit, which are included in the internal boosted voltage-generating circuit depicted in FIG. 4; FIG. 6 is a circuit diagram showing a detailed configuration of a booster circuit included in the internal boosted voltage-generating circuit depicted in FIG. 4; FIG. 7 is a circuit diagram showing a detailed configuration of a level conversion circuit employed in the DRAM depicted in FIG. 3; FIG. 8A is a circuit diagram showing a detailed configuration of the reference voltage-generating circuit of the internal boosted voltage-generating circuit depicted in FIG. 3; FIG. 8B is a circuit diagram showing another detailed configuration of the reference voltage-generating circuit of the internal boosted voltage-generating circuit depicted in FIG. 3; FIG. 8C is a circuit diagram showing still another detailed configuration of the reference voltage-generating circuit of the internal boosted voltage-generating circuit depicted in FIG. 3; FIG. 8D is a circuit diagram showing still another detailed configuration of the reference voltage-generating circuit of the internal boosted voltage-generating circuit depicted in FIG. 3; FIG. 9 is a circuit diagram showing part of a DRAM according to the second embodiment of the present invention; FIG. 10 is a circuit diagram showing part of a shared sense amplifier type DRAM according to the third embodiment of the present invention; FIG. 11 is a circuit diagram showing part of a DRAM according to the fourth embodiment of the present invention; FIG. 12 is a timing chart showing one of the manners in which the DRAM depicted in FIG. 11 operates; FIG. 13 is a circuit diagram showing a specific configuration of a word line-selecting circuit employed in the DRAM depicted in FIG. 11; FIG. 14 shows how the circuits of the DRAM depicted in FIG. 11 are arranged with reference to a semiconductor substrate; FIG. 15 shows one of the manners in which output signals from the level conversion circuit depicted in FIG. 14 are produced; and FIG. 16 is a timing chart of the signals indicated in FIG. 15. Embodiments of the present invention will now be described with reference to the accompanying drawings. FIG. 3 is a circuit diagram showing an equalizing circuit and its peripheral circuits and illustrating an application of the present invention to a DRAM. The DRAM is formed on a semiconductor substrate. In FIG. 3, the portions corresponding or similar to those of the prior art shown in FIG. 1 are denoted by the same reference numerals as used in FIG. 1. As shown in FIG. 3, an equalizing circuit 14, made up of three N-channel MOS transistors 11, 12 and 13, is connected between a pair of bit lines BL and BL . The source-drain path of MOS transistor 11 is connected between one of the bit lines (namely, bit line BL) and a node to which a pre-charging potential VBL is applied. The source-drain path of MOS transistor 12 is connected between the other one of the bit lines (namely, bit line BL ) and the node to which the pre-charging potential VBL is applied. The source-drain path of MOS transistor 13 is connected between the bit lines BL and BL . An equalization control signal EQL is supplied to the gates of the three MOS transistors 11, 12 and 13. Although not shown, memory cells and dummy cells are connected to the bit lines BL and BL . When data is read out, the memory cells and dummy cells are selected by word lines (not shown) and dummy word lines (not shown), and a potential corresponding to the storage data of a selected memory cell and a dummy memory cell is read out to the bit lines BL and BL . It should be noted that the dummy memory cells and dummy word lines are not absolutely required. A sense amplifier circuit 15 is connected to the bit lines BL and BL . After data is read out, the sense amplifier circuit 15 amplifies the potential difference between the bit lines BL and BL , so as to detect the data. The equalization control signal EQL is output from a level conversion circuit 16. An internal boosted voltage-generating circuit 17 constantly generates a boosted voltage Vint which is higher in level than the externally-applied power supply voltage Vcc applied to a power supply terminal 10. The boosted voltage Vint is applied to the level conversion circuit 16. The level conversion circuit 16 converts input control signal EQL' (the high level voltage of which is equal to or lower than the externally-applied power supply voltage Vcc) into the boosted voltage Vint, thereby generating the equalization control signal EQL. Therefore, the high-level voltage of the equalization control signal EQL generated by the level conversion circuit 16 is as high as the boosted voltage Vint, and the low-level voltage thereof is a grounding voltage equal to input control signal EQL'. In the equalizing circuit 14, the three MOS transistors 11, 12 and 13 are turned on in response to the equalization control signal EQL, and the bit lines BL and BL are pre-charged by the pre-charging potential VBL and are then short-circuited to each other to the same potential level. Since, at the time, the voltage of the equalization control signal EQL is sufficiently higher than the externally-applied power supply voltage Vcc, no potential difference is produced between the source and drain of each of the MOS transistors 11, 12 and 13. Hence, the bit lines are set to have the predetermined pre-charging potential VBL. In this embodiment, the voltage of a signal need not be temporarily raised by means of a boot-strap circuit, as in the prior art. Therefore, complex timing control is not required. After the equalizing operation by the equalizing circuit 14, a memory cell and a dummy memory cell are selected by a word line and a dummy word line, and the potential corresponding to the storage data of the selected memory cell and dummy memory cell is read out to the bit lines BL and BL . After the data is read out, the sense amplifier circuit 15 amplifies the potential difference between the bit lines BL and BL , so as to detect the data. FIG. 4 is a block diagram showing the configuration of the internal boosted voltage-generating circuit 17 used for constantly generating a boosted voltage Vint which is higher in level than the externally-applied power supply voltage Vcc. The internal boosted voltage-generating circuit 17 is made up of: a reference voltage-generating circuit 21, a booster circuit 22, a voltage conversion circuit 23, and a voltage comparison circuit 24. The reference voltage-generating circuit 21 generates a constant-value reference voltage 1 by utilization of the externally-applied power supply voltage Vcc. The booster circuit 22 generates a boosted voltage Vint which is higher than the externally-applied power supply voltage Vcc. The voltage conversion circuit 23 generates a voltage 2 which is lower than the boosted voltage Vint generated by the booster circuit 23, by utilization of the boosted voltage Vint. The voltage comparison circuit 24 compares the reference voltage 1 generated by the reference voltage-generating circuit 21 with the voltage 2 generated by the voltage conversion circuit 23, and produces a control signal 3 on the basis of the comparison. The control signal 3 is supplied to the booster circuit 22 to control the boosting operation. In the internal boosted voltage-generating circuit 17, the booster circuit 22, the voltage conversion circuit 23 and the voltage comparison circuit 24 jointly form a closed loop. In accordance with this closed loop, the boosted voltage Vint is controlled to have a constant value which is proportional to the reference voltage 1. Each of the circuits of the internal boosted voltage-generating circuit 17 will be described in detail. FIG. 5 shows detailed configurations of the reference voltage-generating circuit 21, voltage conversion circuit 23 and voltage comparison circuit 24 of the internal boosted voltage-generating circuit 17. The reference voltage-generating circuit 21 is a circuit for generating a voltage which has little voltage dependence on the externally-applied power supply voltage Vcc and which has little temperature dependency. Various types of circuits can be employed as the reference voltage-generating circuit 21, such as a band gap reference circuit using bipolar transistors, or a circuit for generating a substantially-constant voltage by means of MOS transistors having no implanted channel ions. In the circuit arrangement shown in FIG. 5, a band gap reference circuit is employed as the reference voltage-generating circuit 21. The reference voltage-generating circuit 21 depicted in FIG. 5 is made up of a constant current source 31, three NPN bipolar transistors 32, 33 and 34, and three resistors 35, 36 and 37. The reference voltage-generating circuit 21 produces a reference voltage 1 which is stable and has no temperature dependence by adding the emitter-base voltage V1 of a bipolar transistor having a negative temperature coefficient to the dropping voltage V2 appearing in resistor 35. Resistor 35 has a positive temperature coefficient and is formed by utilizing the fact that the temperature coefficient of the emitter-base voltage V1 varies in accordance with the current density of the emitter current of that bipolar transistor. The voltage conversion circuit 23 is made up of two resistors 38 and 39 connected in series between a node to which the boosted voltage Vint is applied and a node to which the grounding potential is applied. The voltage obtained by the voltage conversion circuit 23 has a value corresponding to the resistance ratio of the two resistors 38 and 39. The voltage comparison circuit 24 is a known type of voltage comparison circuit and includes two P-channel MOS transistors 41 and 42 and two N-channel MOS transistors 43 and 44. Voltages 1 and 2 described above are applied to the gates of the N-channel MOS transistors 43 and 44, respectively. Signal 3 which is based on the levels of voltages 1 and 2 is output from the connection node located between MOS transistors 42 and 44. FIG. 6 shows a detailed configuration of the booster circuit 22 identified above. FIG. 6 shows a charge pump type booster circuit, although a variety of types of booster circuits may be utilized in the present invention. As shown, the charge pump type booster circuit is made up of a clock oscillator 51, a buffer circuit 52, and a charge pump circuit 53. The clock oscillator 51 shown in FIG. 6 is a 5 stage-ring oscillator, which is one of the simplest types of oscillators. The clock oscillator 51 comprises five CMOS inverters 61, 62, 63, 64 and 65 connected in multiple stages, and each of the CMOS inverters 61, 62, 63, 64 and 65 is made up of a P-channel MOS transistor and an N-channel MOS transistor. An output of the final-stage inverter 65 is fed back to the first-stage inverter 61. An N-channel MOS transistor 66, the gate of which is applied with the output signal 3 of the voltage comparison circuit 24, has its source-drain path connected between the source of the N-channel MOS transistor of the first-stage inverter 61 and a node to which the ground potential is applied. A P-channel MOS transistor 67, the gate of which is applied with output signal 3, has its source-drain path connected between a node to which the gates of the P- and N-channel MOS transistors of the second-stage inverter 62 are connected in common and a node to which the externally-applied power supply voltage Vcc is applied. An output of an arbitrary-stage inverter, e.g., the third-stage inverter 63, is supplied to the buffer circuit 52. This buffer circuit 52 is made up of two inverters 68 and 69 connected in series, and an output of the buffer circuit 52 is supplied to the charge pump circuit 53. The charge pump circuit 53 is a known type of charge pump circuit and includes a capacitor 70, and two diodes 71 and 72. The two diodes 71 and 72 may be replaced with MOS transistors connected as diodes. When, in the booster circuit of the above configuration, the output signal 3 of the voltage comparison circuit 24 is "L" in level, the N-channel MOS transistor 66 connected to the first-stage inverter 61 of the clock oscillator 51 (FIG. 6) is in the OFF state. Since no current flows through the inverter 61, the clock oscillator 51 does not perform oscillation. At a time, the P-channel MOS transistor 67 connected to the second-stage inverter 62 is in the ON state, and the initial value of the signal input to the second-stage inverter 62 is set to be "H" in level. The clock oscillator 51 starts operating when signal 3 becomes "H" in level. To be more specific, when signal 3 becomes "H" in level and the N-channel MOS transistor 66 is turned on, the first-stage inverter 61, to which the MOS transistor 66 is connected, is rendered operative. Since, at a time, the initial value of the input signal to the second-stage inverter 62 is "H" in level and the output signal from the final-stage inverter 65 is "H" in level, the output signal of the final-stage inverter 65 is supplied to the second-stage inverter 62 after being inverted. Since the P-channel MOS transistor 66, which is for setting the initial value, is in the OFF state then, input signals to the second-stage inverter 62 are sequentially inverted, thus causing an oscillation operation. Therefore, MOS transistor 66 serves as a switch for controlling the oscillation operation, and MOS transistor 67 serves as a switch for determining the initial value of each inverter when the oscillation operation stops. It should be noted that MOS transistor 67 is not absolutely required and can be omitted, if so desired. The buffer circuit 52 is employed for producing a current which is sufficient for driving the capacitor 70 of the charge pump circuit 53 in response to the oscillation signal obtained by the clock oscillator 51. In the charge pump circuit 53, diode 71 permits positive charge to be supplied from the node of the externally-applied power supply voltage Vcc to the capacitor 70 when the output signal from the buffer circuit 52 changes from "H" (i.e., the Vcc level) to "L" (i.e., the ground level), and prevents the flow of charge when the output signal from the buffer circuit 52 changes from "L" to "H". Likewise, diode 72 prevents the flow of charge from the node of voltage Vint when the output signal from the buffer circuit 52 changes from "H" to "L", and permits positive charge to flow to the node of voltage Vint when the output signal from the buffer circuit 52 changes from "L" to "H". Hence, positive charge flows from the node of voltage Vcc to the node of voltage Vint, with the result that voltage Vint becomes higher than voltage Vcc. The charge pump circuit 53 shown in FIG. 6 is one of the simplest configurations. A boosted voltage may be generated by means of a charge pump circuit of another configuration. For example, a charge pump circuit controlled by a 2-phase clock signal may be employed. FIG. 7 shows a detailed configuration of the level conversion circuit 16 depicted in FIG. 3. The level conversion circuit 16 is applied with the boosted voltage Vint as the power supply voltage. The level conversion circuit 16 comprises differential amplifiers, one of which is made up of two P-channel MOS transistors 81 and 82, and the other of which is made up of two N-channel MOS transistors 83 and 84. When control signal EQL' (which is a signal before the level conversion) and an inverted signal EQL' of control signal EQL' are supplied to the gates of the N-channel MOS transistors 83 and 84, signal EQL, the high-level voltage of which is level-shifted to the boosted voltage Vint is output from the connection node located between MOS transistors 82 and 84. FIGS. 8A to 8D are circuit diagrams each showing a detailed configuration of the reference voltage-generating circuit 21, the configurations shown in FIGS. 8A to 8D being different from that depicted in FIG. 5. The circuit shown in FIG. 8A comprises a resistor 91 and n diodes 92. In the circuit shown in FIG. 8A, the value of voltage 1 is determined by a voltage n times higher than the forward voltage Vf of each diode and the equivalent ON-state resistance of each diode. The circuit shown in FIG. 8B is obtained by replacing the diodes 92 depicted in FIG. 8A with P-channel MOS transistors 93 connected as diodes. In the case shown in FIG. 8B, the value of voltage 1 is determined by a voltage n times higher than the threshold voltage Vth of each MOS transistor 93 and the equivalent ON-state resistance of each transistor. The circuit shown in FIG. 8C is obtained by replacing the resistor 91 depicted in FIG. 8B with a P-channel MOS transistor 94. The circuit shown in FIG. 8D is obtained by replacing P-channel MOS transistors 94 and 93 depicted in FIG. 8C with N-channel MOS transistors 95 and 96, respectively. As described above, various circuit configurations can be adopted for the reference voltage-generating circuit 21. It should be noted that the reference voltage 1 generated by the reference voltage-generating circuit 21 may be subjected to voltage conversion by a voltage conversion means of resistance division type, and a voltage obtained by the voltage conversion may be used as a reference voltage. This can be applied to the case of the above-mentioned voltage conversion circuit 23 as well. When bit lines are pre-charged by means of the equalizing circuit 14 shown in FIG. 3, the gate capacitances of MOS transistors 11, 12 and 13 of the equalizing circuit 14 are connected to signal EQL. In other words, the gate capacitances serve to suppress the potential variations of signal EQL. Therefore, an additional capacitor need not be connected to the output terminal of the charge pump circuit 53, so as to suppress the potential variation of signal EQL, and the chip size can be reduced accordingly. FIG. 9 shows a DRAM according to the second embodiment of the present invention. The DRAM of the second embodiment differs from that of the embodiment shown in FIG. 3 in that the equalizing circuit 14 is made up of only two N-channel MOS transistors 11 and 12 (that is, N-channel MOS transistor 13 is not employed). In the equalizing circuit 14 of the configuration of the second embodiment, the two MOS transistors 11 and 12 are turned on in response to the equalization control signal EQL, and the bit lines BL and BL are pre-charged in accordance with the pre-charging voltage VBL. Further, the potentials of the two bit lines are equalized by the MOS transistors 11 and 12. As can be seen from the above, the MOS transistors 11 and 12 employed in the second embodiment have both a pre-charging function and an equalizing function. Not only in the first embodiment but also in the second embodiment, the voltage value of the equalization control signal EQL is sufficiently higher than the externally-applied power supply voltage. Therefore, no potential difference is produced between the source and drain of each of the MOS transistors 11 and 12, and the bit line potentials can be set to the predetermined pre-charging potential VBL. The second embodiment is advantageous in that one MOS transistor can be omitted from such an equalizing circuit 14 as employed in the embodiment shown in FIG. 3. Therefore, the chip size can be reduced at the time of integration. FIG. 10 shows the third embodiment of the present invention. In this embodiment, the present invention is applied to a shared sense amplifier type DRAM as is shown in FIG. 2. FIG. 10 shows equalizing circuits and their peripheral circuits. In FIG. 10, those circuit elements which correspond or are similar to those shown in FIG. 2 are denoted by the same reference numerals as used in FIG. 2, and a detailed description of such circuit elements will be omitted. The equalization control signals EQL1 and EQL2 and switch control signals T1 and T2, which are used in the DRAM of the third embodiment, are generated by level conversion circuits 16a, 16b, 16c and 16d. Each of these level conversion circuits 16a, 16b, 16c and 16d is applied with a boosted voltage Vint. The boosted voltage Vint is higher than the externally-applied power supply voltage Vcc and is constantly generated by an internal boosted voltage-generating circuit 17 of a similar circuit configuration to that shown in FIG. 4. The level conversion circuits 16a, 16b, 16c and 16d respectively convert signals EQL1', T1', T2' and EQL2' (whose high-level voltages are equal in level to the externally-applied power supply voltage Vcc) into the boosted voltages Vint described above. By this conversion, equalization control signals EQL1' and EQL2' and switch control signals T1' and T2' are generated. In the DRAM of the third embodiment, the bit lines can be set to the predetermined pre-charging level when the bit lines are pre-charged by means of the equalizing circuits 14a and 14b, as in the embodiment shown in FIG. 3. In addition, complex timing control is not required, as in the conventional case where the voltage of a signal is temporarily raised by means of a boot-strap circuit. Further, since the DRAM of the third embodiment uses boosted voltages Vint as switch control signals T1 and T2, no potential difference is produced between the source and drain of each of the MOS transistors 19 and 20 of switch circuits 18a and 18b, when a sense amplifier circuit 15 performs data detection, with the MOS transistors 19 and 20 being ON. Hence, the sensing margin is not adversely affected. The DRAM shown in FIG. 10 was described, referring to the case where both equalization control signals EQL1, EQL2 and switch control signals T1, T2 were generated by means of the level conversion circuits. Needless to say, however, either the equalization control signals EQL1, EQL2 or switch control signals T1, T2 may be generated by means of the level conversion circuits. FIG. 11 shows a DRAM according to the fourth embodiment of the present invention. Like the DRAMS described above, the DRAM of the fourth embodiment is formed on a semiconductor substrate. In FIG. 11, those circuit elements which correspond or are similar to those shown in FIG. 3 are denoted by the same reference numerals as used in FIG. 3, and a detailed description of such circuit elements will be omitted. Referring to FIG. 11, an equalizing circuit 14 is connected between a pair of bit lines BL and BL . The equalizing circuit 14 is made up of three N-channel MOS transistors 11, 12 and 13, and each of the N-channel MOS transistors 11, 12 and 13 is controlled by an equalization control signal EQL. Two memory cells MC are connected to the bit lines BL and BL , respectively. Each of the memory cells MC is made up of a selecting transistor T and a charge storage capacitor C. At the time of reading out data, a memory cell is selected by a signal on one of word lines WL, and a potential corresponding to the charge stored in the capacitor of the selected memory cell MC is read out to either bit line BL or bit line BL . The potential difference between the bit lines BL and BL is amplified by a sense amplifier circuit 15, for the detection of the data. The equalization control signal EQL is output from a level conversion circuit 16. An internal boosted voltage-generating circuit 17 constantly generates a boosted voltage Vint which is higher in level than the externally-applied power supply voltage Vcc, and the boosted voltage Vint is applied to the level conversion circuit 16. The level conversion circuit 16 converts signal EQL' (the high-level voltage of which is equal to or lower than the externally-applied power supply voltage Vcc) into the boosted voltage Vint, thereby generating the equalization control signal EQL. Therefore, the high-level voltage of the equalization control signal EQL generated by the level conversion circuit 16 is as high as the boosted voltage Vint, and the low-level voltage thereof is a ground voltage equal to signal EQL' (i.e., a signal which has not yet been converted). A word line-selecting circuit 19 selects one of the word lines WL on the basis of row address signals. The word line-selecting circuit 19 is made up of a pre-decoder 19a, a row decoder 19b, and a word line driver 19c. The boosted voltage Vint generated by the internal boosted voltage-generating circuit 17 is applied to the word line-selecting circuit 19, and the high-level voltage of a signal supplied to the word line WL from the word line-selecting circuit 19 is equal to the boosted voltage Vint. In the DRAM of the above configuration, the equalization control signal EQL is made to become a high level before data is read out from a memory cell MC, as shown in the timing chart depicted in FIG. 12. The equalizing circuit 14 pre-charges the bit lines BL and BL and then short-circuits them to each other to the same potential level VBL. Since, at a time, the voltage of the equalization control signal EQL is raised to the boosted voltage Vint and is sufficiently higher than the externally-applied power supply voltage Vcc, no potential difference is produced between the source and drain of each of the MOS transistors 11, 12 and 13 of the equalizing circuit 14. Hence, the bit lines are set to have the predetermined pre-charging potential VBL. In the data readout operation which is subsequent to the bit line-equalizing operation, the word line-selecting circuit 19 selects and drives one of the word lines WL on the basis of row address signals, as shown in the timing chart of FIG. 12. At a time, the high-level voltage of a signal supplied to the selected word line WL is equal to the boosted voltage Vint generated by the internal boosted voltage-generating circuit 17. Therefore, the selecting transistor T of the selected memory cell MC is in the fully-conductive state, thus shortening the time required for reading out the charge from the capacitor C. As described above, in the embodiment shown in FIG. 11, the boosted voltage Vint generated by the internal boosted voltage-generating circuit 17 is used by both the level conversion circuit 16 (which is for the equalization of the bit line potentials) and the word line-selecting circuit 19 (which is for the word line selection). As shown in FIG. 12, the period of time in which a word line WL is driven (i.e., the period of time in which the word line WL is at the high level) and the period of time in which the bit line potentials are equalized (i.e., the period of time in which signal EQL is at the high level) do not overlap with each other. Therefore, the single internal boosted voltage-generating circuit 17 can be used in common by circuits 16 and 19. FIG. 13 shows a detailed configuration of each of the pre-decoder 19a, row decoder 19b, and word line driver 19c of the word line-selecting circuit 19 employed in the DRAM depicted in FIG. 11. The pre-decoder 19a is made up of: a level shift circuit (LS) 111 for shifting the level of the high-level voltage of a pre-charging control signal PRE (which is derived from the node of voltage Vcc) to the level of the boosted voltage Vint; a P-channel MOS transistor 113 for controlling a pre-charge node 112 to have a pre-charging potential on the basis of an output from the level shift circuit 111; a number of decoding N-channel transistors 114, connected in series, for determining whether or not the pre-charge node 112 should be discharged, on the basis of some of the row address signals; and a CMOS inverter 115 for inverting the signal appearing at the pre-charge node 112. The row decoder 19b is made up of: a level shift circuit (LS) 121 for shifting the level of the high-level voltage of a pre-charging control signal PRE (which is derived from the node of voltage Vcc) to the level of the boosted voltage Vint; a P-channel MOS transistor 123 for controlling a pre-charge node 122 to have a pre-charging potential on the basis of an output from the level shift circuit 121; a number of decoding N-channel transistors 124, connected in series, for determining whether or not the pre-charge node 122 should be discharged, on the basis of some of row address signals. The word line driver 19c is connected between the output terminal of the pre-decoder 19a and a node of ground potential. The word line driver 19c comprises a CMOS inverter 131 to which an output of the row decoder 19b is supplied. The word line-selecting circuit of the above configuration operates as follows. When some of the row address signals supplied to the pre-decoder 19a satisfy a predetermined logical condition, the pre-decoder 19a outputs the boosted voltage Vint. This boosted voltage Vint is applied to the corresponding word line WL by way of the word line driver 19c when remain of the row address signals supplied to the row decoder 19b satisfy a predetermined logical condition. In this manner, memory cell MC is selected. FIG. 14 shows how the circuit elements of the DRAM shown in FIG. 11 are arranged on a semiconductor substrate. Referring to FIG. 14, memory cells, each made up of a selecting transistor and a capacitor, are grouped into a number of memory cell arrays (CA) 150. The memory cell arrays 150 are arranged in one line. Equalizing circuit 14a and switch circuit 18a are located on one side of each memory cell array 150, and equalizing circuit 14b and switch circuit 18b are located on the other side. A sense amplifier (SA) 15 is located between the adjacent memory cell arrays 150. A group of wiring lines 151, including a wiring line 151a to which the boosted voltage Vint is applied and a number of wiring lines 151b to which row address signals are supplied, extend parallel to one another in the direction in which the memory cell arrays 150, sense amplifier circuits 15, equalizing circuits 14a, 14b, and switch circuits 18a, 18b are arranged. Level conversion circuits 153, including the level conversion circuits 16a to 16c described above, and the word line-selecting circuits 19 are arranged in one line such that they are located between the group 151 of wiring lines and the arrangement of the memory cell arrays 150, sense amplifiers 15, equalizing circuits 14a, 14b, and switch circuits 18a, 18b. In the DRAM shown in FIG. 14, the memory cell arrays 150, the level conversion circuits 153 and the word line-selecting circuits 19 are arranged adjacent to the group 151 of wiring lines, including the wiring line 151a to which the boosted voltage Vint is applied. The sources of the P-channel MOS transistors of these circuits are applied with the boosted voltage Vint. In order to permit a forward current to flow through the PN junctions formed by the sources and back gates of the P-channel MOS transistors, it is necessary to apply the boosted voltage Vint to N-wells of the P-channel MOS transistors. Since, in the DRAM shown in FIG. 14, the memory cell arrays 150, the level conversion circuits 153 and the word line-selecting circuits 19 are arranged adjacent to the group 151 of wiring lines, a single N-well can be used for the P-channel MOS transistors, and the boosted voltage Vint can be applied to this single N-well. Accordingly, the chip size can be small in comparison with the case where a number of N-wells must be provided. FIG. 15 shows one of the manners in which output signals EQL1, EQL2, T1 and T2 from one of the level conversion circuits 153 are produced in the DRAM shown in FIG. 14, and FIG. 16 is an example of a timing chart of output signals EQL1, EQL2, T1 and T2. The various semiconductor memory devices described hereinabove enable easy control of the timings at which either an operation for equalizing potentials of bit lines or an operation for controlling the connection between the bit lines and a sense amplifier is performed. In addition, they ensure high-speed operation. Further, they enable bit lines to be pre-charged to a predetermined potential level, as in the conventional art, without adversely affecting the sensing margin.

Claims: A semiconductor memory device comprising: first and second bit lines (BL, BL ); a boosted voltage generating circuit (17) for generating a boosted voltage (Vint) having a voltage level higher than a voltage level of a power supply voltage (Vcc) supplied to an input thereof; and a charging/equalizing circuit (14) having at least one transistor and responsive to a charging/equalizing control signal (EQL) for charging said first and second bit lines with a charging voltage (VBL) and for equalizing said first and second bit lines (BL, BL ) at the same equalized voltage level; characterized in that: the boosted voltage generating circuit (17) is arranged to generate said boosted voltage (Vint) constantly and the charging/equalizing control signal (EQL) has a first voltage level of the boosted voltage (Vint) supplied from said boosted voltage generating circuit (17) which includes a reference voltage generating (21) circuit for generating a reference voltage (1); a boosting circuit (22) responsive to a boosting circuit control signal (3) for generating the boosted voltage (Vint) by boosting the power supply voltage (Vcc) ; a voltage converting circuit (23) for converting the boosted voltage (Vint) to a converted voltage (2)having a voltage level lower than the voltage level of the boosted voltage (Vint); and a voltage comparing circuit (24) for comparing the reference voltage (1) and the converted voltage (2) to generate a comparison signal, and for outputting the comparison signal as the boosting circuit control signal (3). A semiconductor memory device according to claim 1, characterized in that said charging/equalizing circuit (14) includes: a first MOS transistor (11) having a source-drain path connected between said first bit line and a node which is applied with the charging voltage (VBL), and a gate which is supplied with the charging/equalizing control signal (EQL); and a second MOS transistor (12) having a source-drain path connected between said second bit line and said node, and a gate which is supplied with the charging/equalizing control signal (EQL). A semiconductor memory device according to claim 2, characterized in that said first and second MOS transistors (11, 12) have the same channel conductivity type. A semiconductor memory device according to claim 2, characterized by a third MOS transistor (13) having a source-drain path connected between said first and second bit lines, and a gate which is supplied with the charging/equalizing control signal (EQL). A semiconductor memory device according to claim 4, characterized in that said third MOS transistor (13) has the same channel conductivity type as the first and second MOS transistors (11, 12). A semiconductor memory device according to claim 1 or 4, characterized in that said charging/equalizing control signal (EQL) has a second voltage level equal to a ground level. A semiconductor memory device according to any preceding claim, characterized by further comprising a level converting circuit (16) for converting an input control signal (EQL') at a first voltage level into a converted control signal at the voltage level of said boosted voltage (Vint), and for outputting the converted control signal as said charging/equalizing control signal (EQL). A semiconductor memory device according to claim 1, wherein said first and second bit lines form a first pair (BL1, BL1 ) and said charging/equalizing circuit (14) is a first charging/equalizing circuit, said semiconductor memory device further comprising: second bit line pairs (BL2, BL2 ) having first and second bit lines; a sense amplifier (15) for sensing and amplifying a potential difference between the first and second bit lines of said first and second bit line pairs; a second charging/equalizing circuit (14b), responsive to a second charging/equalizing control signal (EQL2) having a first voltage level of the boosted voltage (Vint) (supplied from said boosted voltage generating circuit (17)), for charging said first and second bit lines of said second bit line pair with a second charging voltage (VBL), and for equalizing said first and second bit lines of said second bit line pair at the same second equalized voltage level; a first switch (18a), responsive to a first switch control signal (T1) having a first voltage level equal to the voltage level of the boosted voltage (Vint), for selectively connecting said first bit line pair (BL1, BL1 ) to said sense amplifier (15); and a second switch (18b), responsive to a second switch control signal (T2) having a first voltage level equal to the voltage level of the boosted voltage (Vint), for selectively connecting said second bit line pair (BL2, BL2 ) to said sense amplifier (15). A semiconductor memory device according to claim 1, characterized in that: said first charging/equalizing control signal (EQL1) is equalized to the voltage level of the boosted voltage (Vint) when the first and second bit lines (BL1, BL1 ) of said first bit line pair are being charged with the first charging voltage (VBL); said second charging/equalizing control signal (EQL2) is equalized to the voltage level of the boosted voltage (Vint) when the first and second bit lines (BL2, BL2 ) of said second bit line pair are being charged with the second charging voltage (VBL); and the first and second switch control signals (T1, T2) are equalized to the voltage level of the boosted voltage (Vint) during a period of time other than when the first and second bit lines of said first and second bit line pairs are being charged with the first and second charging voltages (VBL), respectively. A semiconductor memory device according to claim 1, characterized in that said first charging/equalizing circuit (14a) includes: a first MOS transistor (11) having a source-drain path connected between the first bit line ( BL1 ) of said first bit line pair and a first node which is supplied with the first charging voltage (VBL), and a gate which is supplied with the first charging/equalizing control signal (EQL1); a second MOS transistor (12) having a source-drain path connected between the second bit line (BL1) of said first bit line pair and said first node, and a gate which is supplied with the first charging/equalizing control signal (EQL1); and a third MOS transistor (13) having a source-drain path connected between the first and second bit lines (BL1, BL1 ) of said first bit line pair, and a gate which is supplied with the first charging/equalizing control signal (EQL1); and a said second charging/equalizing circuit (14b) includes: a fourth MOS transistor (11) having a source-drain path connected between the first bit line (BL2) of said second bit line pair and a second node which is supplied with the second charging voltage (VBL), and a gate which is supplied with the second charging/equalizing control signal (EQL2); a fifth MOS transistor (12) having a source-drain path connected between the second bit line ( BL2 ) of said second bit line pair and said second node, and a gate which is supplied with the second charging/equalizing control signal (EQL2); and a sixth MOS transistor (13) having a source-drain path connected between the first and second bit lines (BL2, BL2 ) of said second bit line pair, and a gate which is supplied with the second charging/equalizing control signal (EQL2). The semiconductor memory device according to claim 10, characterized in that said first, second, third, fourth, fifth and sixth MOS transistors (11, 12, 13) have the same channel conductivity type. A semiconductor memory device according to claim 1, characterized in that said first charging/equalizing control signal (EQL1) has a second voltage level equal to a ground voltage and said second charging/equalizing control signal (EQL2) has a second voltage level equal to the ground voltage. A semiconductor memory device according to claim 1, 4, 5 or 9, characterized by further comprising: memory cells (MC) connected to said first and second bit lines (BL, BL ); word lines (WL) for selecting said memory cells (MC) in accordance with a word line driving signal; a level converting circuit (16) for converting an input control signal (EQL') at a first voltage level into a converted control signal at the voltage level of the boosted voltage (Vint), and for outputting the converted control signal as the charging/equalizing control signal (EQL); and word line selecting/driving circuit (19), responsive to address signals for generating selection signals having a first voltage level equal to the voltage level of the boosted voltage (Vint), and for outputting the selection signals as the word line driving signals. A semiconductor memory device according to claim 13, characterized in that said selection signals are row address signals. A semiconductor memory device according to claim 14, characterized by further comprising: a plurality of address lines (151b) for supplying said row address signals; and a voltage line (151a), extending in a direction parallel to said plurality of address lines (151b), for supplying the boosted voltage (Vint). A semiconductor memory device according to claim 13, characterized in that a period of time when the charging/equalizing signal (EQL) has a voltage level equal to the voltage level of the boosted voltage (Vint) do not overlap with a period of time when the selection signals (WL) have a voltage level equal to the voltage level of the boosted voltage. A semiconductor memory device according to claim 13, characterized in that said word line selecting/driving circuit (19) includes: row decoders (19b), corresponding in number to the word lines (WL), for decoding some of the address signals; pre-decoders (19a), corresponding in number to the word lines (WL), for decoding remaining ones of the address signals; and word line drivers (19c), corresponding in number to the word lines (WL), for outputting decoder outputs of the pre-decoders (19a) as said word line driving signals in accordance with decoder outputs of the row decoders (19) . A semiconductor memory device according to claim 17, characterized in that each of said row decoders (19) includes: a first transistor (123), which is of a first polarity, for pre-charging an output node (122) to a voltage level equal to the voltage level of the boosted voltage (Vint) on the basis of a pre-charging control signal ( PRE ); and a plurality of second transistors (124) which are of a second polarity and respectively have source-drain path and gate, said source-drain paths being connected in series between the output node (122) and a ground potential node, and the gates receiving the address signals. A semiconductor memory device according to claim 17, characterized in that each of said pre-decoders (19a) includes: a third transistor (113), which is of a first polarity, for pre-charging an output node (112) to a voltage level equal to the voltage level of the boosted voltage (Vint) on the basis of a pre-charging control signal ( PRE ); and a plurality of fourth transistors (114) which are of a second polarity and respectively have source-drain path and gate, said source-drain paths being connected in series between the output node (112) and a ground potential node, and the gates receiving the address signals. A semiconductor memory device according to claim 17, characterized in that each of said word line drivers (19c) is an inverter circuit (131), said inverter circuit (131) being supplied with a decoder output of the pre-decoder (19a) as one power supply potential and a ground potential as another power supply potential, and further supplied with a decoder output of the row decoder (19b). A semiconductor memory device according to claim 1, comprising: a plurality of memory cell arrays (150) arranged in a line, each of said memory cell arrays including a plurality of memory cells which are connected to said first and second bit lines (BL, BL ) and to word lines (WL) ; a plurality of sense amplifiers (15) for sensing data stored in the memory cells of the memory cell arrays (150), each of said sense amplifiers (15) being located between adjacent two of the memory cell arrays (150); a plurality of switch circuits (18a, 18b) for transferring data from the memory cells of the memory cell arrays (150) to the sense amplifiers (15), each of said switch circuits (18a, 18b) being located between one of the memory cell arrays (150) and an adjacent one of the sense amplifiers (15); and a plurality of charging/equalizing circuits (14a, 14b), each responsive to a charging/equalizing signal, for charging the first and second bit lines (BL, BL ) with a charging voltage and for equalizing the first and second bit lines at the same equalized voltage level, each of the charging/equalizing circuits (14a, 14b) being located between one of the memory cell arrays (150) and an adjacent one of the sense amplifiers (15). The semiconductor memory device according to claim 21, characterized by further comprising: a wiring line (151a) for receiving a transmitted boosted voltage (Vint) higher than a voltage level of an input power supply voltage (Vcc) is transmitted; a plurality of word line selecting circuits (19) which are arranged in parallel to an arrangement made up of the memory cell arrays (150), the sense amplifiers (15) and the switch circuits (18a, 18b), said word line selecting circuits (19) being arranged for selecting word lines connected to the memory cell arrays (150) in response to address signals and being applied with the boosted voltage (Vint) transmitted to the wiring line; a plurality of level converting circuits (153), to be applied with the boosted voltage (Vint) transmitted to the wiring line (151a), for converting a first voltage level of an input control signal into a voltage level of the boosted voltage (Vint), and for supplying a control signal obtained thereby to the charging/equalizing circuits (14a, 14b) as said charging/equalizing signal, each of said level converting circuits (153) being located between adjacent ones of the word line-selecting circuits (19). The semiconductor memory device according to claim 22, characterized in that said wiring line (151a) is adjacent to an arrangement made up of the word line selecting circuits (19) and the level converting circuits (153). The semiconductor memory device according to claim 22, characterized by further comprising a plurality of wiring lines (151b) to which the address signals supplied to the memory cell arrays (150) are transmitted, said plurality of wiring lines (151b) extending in parallel to the wiring line (151a) to which the boosted voltage (Vint) is to be transmitted.

Inventors: KANEKO TETSUYA, OHSAWA TAKASHI, KANEKO, TETSUYA, OHSAWA, TAKASHI, KANEKO, TETSUYA, C/O INTELLECTUAL PROPERTY DIV., OHSAWA, TAKASHI, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1994
Main classification: G11C7
IPC codes: G11C5, G11C11, G11C7